The discovery last week of water ice just under the surface of Mars has researchers buzzing, given that water is a key ingredient for life. The finding, by the Phoenix Mars Lander, is the most recent hint that the Red Planet might be habitable to microbes.

But in the parlance of treasure hunters in the movie "National Treasure," this looks a lot like just another clue that will lead to other clues, and still more clues. The big question still hangs over NASA: Is there life on Mars? And just as important: Can NASA ever find the evidence for it?

Getting to that answer will require the right mission with the right tools in the right places — not to mention some serious digging beyond the capabilities of Phoenix. The next Mars missions include NASA's Mars Science Laboratory, an SUV-sized rover set to launch in 2009, and the European ExoMars rover that would wield a drill capable of digging 6.5 feet down. It is set to launch in 2013.

NASA has long taken an incremental approach to searching for biology, with "follow the water" as a driving strategy. That means, perhaps to the frustration of some, that the current Phoenix lander mission and the twin rovers on Mars are not even designed to detect Martian life.

So current and near-future missions may not directly look for life, scientists say, but they will likely turn up more pieces to the puzzle of Mars and where extraterrestrial life may thrive.

Location, location
Scientists generally agree that finding existing or previous life means digging down at least a meter, away from the harmful radiation on the surface of Mars.

The Phoenix lander has scraped away enough of the surface to uncover crumbs of water ice, but its backhoe-like robotic arm cannot reach down far enough to try and find whatever life may exist under the north polar region.

"Phoenix could easily be sitting a meter or two above pay dirt," said Michael Storrie-Lombardi, an astrobiologist who heads the Kinohi Institute in California.

Slideshow: Month in Space: January 2014
A serious search for life would likely move further up towards the Martian north pole, where the presence of ice water that sometimes melts would provide the best chance for life to eke out an existence.

However, the melting and refreezing of water could also erase records of previous life or organic material, scientists say. That presents a dilemma between searching for existing life versus past life.

"We don't know exactly how fast the ice comes and goes," said Jack Farmer, an astrobiologist at Arizona State University. "If it's on a routine basis, then you may end up losing the biosignatures."

Farmer and other scientists may prefer a region that remains frozen year-round, preserving organic material and possible traces of past life in the ice.

That safer bet on finding evidence of past life — or at least preserved organic material — means going to places "that are fairly dry and act like a freeze-dried container," Storrie-Lombardi said.

But "if you really want to go for broke" and search for existing life, Storrie-Lombardi said, you need to dig in places where water may flow.

Besides areas near the Martian north pole, scientists have also examined gullies where liquid water may have bubbled up recently in the planet's history. Finding an active hot spring could lead to finding life similar to extremophile bacteria that can thrive under intense conditions.

"Hot springs are at the top of my list," said Bruce Jakosky, a geologist at the University of Colorado who has worked on Mars missions. "Organisms might not survive and thrive on the surface, but recently exposed hot springs might bring something up from beneath."

Tools of the trade Both NASA's MSL and the European ExoMars rovers can explore the Martian surface more effectively than Phoenix, and the ExoMars rover will also carry a heavy duty drill to dig for biological gold. But both rovers still face the challenge of knowing where to look.

"We don't know what the recipe is for success, or the paradigm for preserving life on Mars if it ever evolved," said John Grotzinger, geologist and MSL project scientist. He emphasized that finding preserved organic material is no cinch, and pointed out the rarity of uncovering fossil records of early life on Earth.

The mega-rover MSL will scoop up dust and drill into rocks to test for organic building blocks of life such as carbon, hydrogen, nitrogen and oxygen. MSL will also carry a laser to vaporize rock samples and allow it to sift through the resulting dust.

Grotzinger and the MSL team aim to boost MSL's chances by choosing a landing site with a diverse environment, including areas with signs of water and minerals such as clay and sulfates that could preserve organic material.

ExoMars holds the additional promise of allowing scientists to dig deeper than ever before on Mars, and presents possibly the best chance of finding solid evidence of existing or past life. However, not knowing what lies beneath the Martian surface tempers optimism for finding life during the near-future missions.

An additional instrument may improve the odds for MSL or ExoMars. Storrie-Lombardi worked with U.S. and U.K. researchers on a small laser that can make organic compounds glow fluorescent under its ultraviolet beam.

The laser probe could illuminate holes dug by ExoMars to see which areas glow the brightest. That would allow scientists to target those places for testing with more limited-supply wet chemistry tests, which mix soil and rock samples with water or other reagents to identify what building blocks of life might exist.

"If the team can show this is light and rugged enough, we will propose taking it to Mars," said Andrew Coates, planetary scientist and ExoMars investigator.

Slow but steadyWhen NASA's Viking mission reached Mars in 1976, the twin landers uncovered no traces of organic material in scooped-up Martian soil. A study later found that the tests used by the Viking landers even failed to detect organic material on Earth in places teeming with microbes.

Much has changed since then besides having more sensitive instruments. Scientists now know much more about the Martian environment and the likelier hiding holes for life, thanks to the long list of Mars missions leading up to Phoenix.

"Phoenix, Mars Science Lander, these all have a chance of discovering life," Jakosky said, "but that's not the goal of the missions."

NASA's incremental approach can appear frustratingly slow to even scientists. But Jakosky explained that each mission has made new and different discoveries that add to overall understanding of Mars as a system — and that helps narrow the search for life.

"Organics can be produced by non-biological processes, and indicators of life on Earth may not be indicators of life on Mars," Jakosky said. "In order to know how to interpret any measures we make, have to look at Mars as a system."

By contrast, Viking represented a shot in the dark without any of that understanding.

"We tried the cash in all your chips approach, and that was the Viking spacecraft," Jakosky said. "It was an incredibly naive, narrow-minded approach, and in hindsight there's no expectation it should have found life."

Bring it homeMSL and ExoMars should help clarify questions about where to find organic material and possibly life on Mars. However, robotic explorers can only provide test results and images to humans back on Earth, which leaves room open for debate about what exactly they have found.

"We might have to wait to bring a sample back or send a human there," Storrie-Lombardi noted. "It's sometimes difficult to come up with concrete evidence that everyone agrees on."

Getting a Mars sample back to Earth would mean scientists could examine the evidence with all the most recent instruments and technology available, instead of just the limited experiments that each Mars mission can carry.

Jakosky agreed that scientists are "probably not getting definitive answer until we bring a sample back," a possible but complicated mission proposal.

ExoMars includes plans that would test technology for such future sample-return missions, and MSL will similarly "take us a long way towards determining what are the favorable rocks to bring back," said Grotzinger on the MSL team.

For now, scientists remain cautious but hopeful about the question of life on Mars — and above all excited.

"I think it'd be stunning if we don't find evidence that there is life or was life in the past," Storrie-Lombardi said.